The present invention relates to a heat exchanger having application for service as a condenser/reboiler of a double distillation column. More particularly, the present invention relates to such a heat exchanger of the type known as a downflow reboiler or a falling film evaporator. Even more particularly, the present invention relates to such a heat exchanger in which a froth is created in a reservoir overlying heat exchange passages. Still even more particularly, the present invention relates to such a heat exchanger in which a liquid phase of the froth is distributed to the heat exchange passages through a distributor tray having orifices sized such that a liquid phase of the froth weeps into the heat exchange passages through the orifices to automatically replenish the heat exchange passages.
Downflow reboilers, also known as falling film evaporators, are used as a vehicle for indirectly transferring heat between a liquid and a vapor. Such heat exchangers are constructed from a plurality of parallel plates to form alternating heat exchange passages to indirectly exchange heat between two fluids such as liquid oxygen and gaseous nitrogen. Often, corrugated fin material is provided within the passages for liquid distribution and heat transfer purposes. Liquid and vapor are alternately distributed to the heat exchange passages so that a falling film of liquid in the liquid passages indirectly exchanges heat with vapor.
Such heat exchangers have application as condenser/reboilers in double distillation column systems. In a double distillation column, a multi-component mixture is fed into a higher pressure distillation column to produce a liquid column bottoms enriched in the higher boiling components and a vapor enriched in the lower boiling components. For instance, in case of low temperature rectification of air the higher boiling component is oxygen and the lower boiling component is nitrogen. The liquid column bottoms is further refined in a lower pressure column operatively associated with the higher pressure column by the condenser/reboiler. A liquid enriched in the higher boiling impurities collects in a sump of the lower pressure column. Reboiler feed produced through distillation in the lower pressure column and also enriched in the higher boiling components engages an indirect heat exchange with the vapor produced as tower overhead in the higher pressure column to vaporize part of the reboiler feed and to condense the vapor. The condensed vapor, in case of air separation, serves to reflux both the higher and lower pressure columns. The reboiler feed, not vaporized through the indirect heat exchange collects as the liquid column bottoms of the sump.
A problem addressed in many prior art designs of such heat exchangers is liquid mal-distribution occasioned by, for instance, leveling inaccuracy. In order to solve the liquid distribution problem, prior art heat exchangers employ distributors in which the liquid is distributed through an ever increasing complex array of openings. For instance in U.S. Pat. No. Re. 33,026, a rough distribution is made through relatively large and widely spaced holes. Thereafter, a finer distribution is made using packing. The problem with the foregoing prior art is that the holes can be easily plugged by solids and there is no self-correcting mechanism to eliminate dry areas. In other prior art heat exchanger designs, liquid distribution systems present a complicated mechanical structure. Installation difficulties further complicate the problem.
As will be discussed, the present invention provides a heat exchanger in which liquid is distributed by a mechanism that is less susceptible to prior art problems and is far simpler in design to liquid distribution systems of the prior art.